Notchback boot/trunk lid as a spoiler

[aerohead]

This had been gestating in my mind and I went ahead and did a workup on it:
In Hucho et al.,SAE Paper 760185, Figure 24, and Hucho et al., 2nd-Edition, Figure 4.62, page 155, they lay out the effect of various aft-body configurations on total drag, for the Golf/ Rabbit, and Jetta.
There's enough visual information in the two figures, with which to reconstruct the actual 'shapes' of the two cars, to complement their associated drag coefficients.
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The Jetta is just the Golf, with a little more than 10-inch ( 254mm ) trunk extension added to the rear.
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An eleven data-point scatter-plot is provided for the Jetta, illustrating the Cd at each configuration, ranging from Cd 0.407, to Cd 0.365.
I drew out six (6) of the configurations.
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* If the 'step' is removed from the Jetta, it produces a 'fastback' with a constant rear slope angle of 33-degrees, placing it in the forbidden region of bistable flow, which on the sibling Golf, generated up to Cd 0.44.
* If the step of the 'Basic Shape' is re-installed, the drag falls to Cd 0.407.
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* The addition of the Basic Shape notchback step is clearly providing the same function as what one would seek from the addition of a rear spoiler.
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* If 150mm of additional height is added to the Basis Shape, the drag falls to Cd 0.37.
* If 450mm is added to the Basic Shape, the drag minimum of Cd 0.365 is achieved, according to the scatter-plot.
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* Adding 550mm to the Basic Shape degrades the flow to Cd 0.38, at Shape B.
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The drag of the Jetta can be altered by delta-Cd 0.075 simply as a function of the rear body slope angle / step height.
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In Hucho's Figure 4.60, by Buchheim et al., the 'curved-roof' wind tunnel model also registers a delta- Cd 0.075, also with the single rear slope angle parameter, by sweeping through 200mm of variable step height, indicating angles from 36-degrees, to 13-degrees.
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If you're out and about, and happen to spy a late-model Accent, Rio, or Mirage notchback sedan, imagine what the rear slope would be without those boot/ trunk lids.

[freebeard]

Do you have figures for the Type III Notchback/Fastback/Squareback?

One could remove the trunk lid from the Notchback and fabricobble an enclosed and raised lid like the Jetta, from the cut-off end of a Squareback roof. It could fit against the stock glass like the cowl induction hood on a Camaro.

[kach22i]

This might help explain the current trend of very high trunk lids on the few coupes and sedans left on the market.

The SUV's and crossover seem to adopted a tapering roof line per the Aero-Template, although there a few hold outs.

[jakobnev]

They forgot to close the trunk lid one day and noticed the car was faster, so they just started propping it open intentionally:

[aerohead]

Originally Posted by freebeard

Do you have figures for the Type III Notchback/Fastback/Squareback?

One could remove the trunk lid from the Notchback and fabricobble an enclosed and raised lid like the Jetta, from the cut-off end of a Squareback roof. It could fit against the stock glass like the cowl induction hood on a Camaro.

I've only got a blueprint for the fastback, and I'm uncertain of its actual Cd. I've seen Cd 0.50 attributed to it. Hucho shows a very small image for the notchback, used as a wind tunnel calibration model, but it's useless when photo-enlarged.
If a modification, in any way changes the relative length of the aft-body, in relation to the length of the car, the new 'roofline' contour would have to be based upon the new relationship.
The Golf/ Rabbit has an active aft-body length of 16.25%.
To create the Jetta, it gets a little over ten ( 10 ) inches added length, increasing the aft-body percentage to 33.43%.
And not so oddly enough, both the Golf and Jetta hit their drag minimums when their aft-body contour passes through the Mair' boat-tail transition / 'slow' template pathway.

[aerohead]

Originally Posted by kach22i

This might help explain the current trend of very high trunk lids on the few coupes and sedans left on the market.

The SUV's and crossover seem to adopted a tapering roof line per the Aero-Template, although there a few hold outs.

Yes.
One easy example is, Audi's E-Tron series. You can watch the progression of drag reduction ( Cd 0.28- Cd 0.26- Cd 0.22 ) as the roofline moves closer to the streamline pathway, from the 'raked-back', to 'Sportback, and finally 'Fastback', on the GT.
Chevy's Malibu MAXX suffered Cd 0.37 from it's mutilated Kamm-back, which dropped to Cd 0.301 when morphed to a notchback.
Current Malibu sedans are almost a true fastback, as CIVIC, Camry, Accord, Optima, Sonata, etc..

[aerohead]

Originally Posted by jakobnev

They forgot to close the trunk lid one day and noticed the car was faster, so they just started propping it open intentionally:

No doubt! The 'Beetle-esque' ( Cd 0.495 ) roofline, meant Cd 0.51 for the Chrysler Airflow, and Cd 0.51 for Citroen's 2CV.
One trick Chrysler used to get the Airflow, DeSoto aerodynamic test mule down to Cd 0.244, was the more 'Kamm-esque' roofline ( 2012 Tesla Model S drag, in 1934 ).

[aerohead]

I've re-visited Figures 4.63- 4.65 in Hucho's 2nd-Edition, and something he said stood out that I thought I'd share:
The context is Buchheim et al.' aero development of the Audi 100-III.
The team worked through wind tunnel investigations of shape optimization for the backlight, boot height, and boot length.
Each component was tweaked until they reached 'saturation', after which, no further improvements were derived from additional change to an angle, height, or length.
Hucho wrote to the effect that, ' The possibilities offered by this measure have not been fully exploited due to the reduction of the view to the rear as well as the appearance of the vehicle from the rear.'
My interpretation of the comment is that, what ended up going into production was short of the aerodynamic 'ultimate', and the decision not to explore lower drag had to do with rearward vision limitation, aesthetics, or constraints governed by designer/ stylist.
We know, for instance, because Buchheim has told us elsewhere that, the 1st-gen Golf / Rabbit, could have had Cd 0.34, instead of Cd 0.42 in 1975, however, VW was under contractual agreement with designer, Giorgetto Giugiaro, to construct the Golf essentially as he had 'drawn' it.
FIAT, in their SAE Paper 860212, also explored Cd as a function of rear slope angle and configuration. Their lowest drag occurred with a notchback with such a 'flattened' backlight angle and high boot that, one would wonder how any driver could ever see out through the rearview mirror. Again, a situation where considerations, other than aerodynamics, dictates the final design.
As we move more and more towards 'synthetic' vision, these issues may play a lesser role in a new vehicle's design specification.